1. Field of the Invention
The present invention relates to low temperature, cationically curable compositions with improved cure speed and toughness.
2. Brief Description of Related Technology
Toughness generally is the ability of a material to absorb energy and undergo large permanent set without rupture. For many engineering adhesive applications, toughness is often the deciding factor. Plastics, because of their inherent brittleness, have heretofore been modified in a variety of ways in efforts to improve the toughness thereof. Epoxy resins, for example, which form a versatile glassy network when cured, exhibit excellent resistance to corrosion and solvents, good adhesion, reasonably high glass transition temperatures (“Tg”) and adequate electrical properties. Unfortunately, however, the poor fracture toughness of epoxy resins oftentimes limits the usefulness thereof in many commercial applications.
The impact strength, as well as other physical properties of crosslinked epoxy resins, is controlled by the chemical structure and molecular weight of the epoxy resin, weight ratio of the epoxy resin to the hardener, by any added fillers, and by the conditions used to cure the formulation. Unfortunately, crosslinked, glassy epoxy resins with a relatively high Tg (>100° C.) are brittle in nature. For instance, a conventional cycloaliphatic epoxy, CYRACURE 6110, cures to a very brittle polymer which shows virtually no T-peel strength. The poor impact strength of high glass transition epoxy resins limits their usage as structural materials and use in or as composites.
Heretofore, cationically curable epoxy compositions have been difficult to toughen due to the high crosslink density and the inherent brittleness of the cured polymer. In addition many conventional toughening agents, such as the CTBN rubbers, contain basic functionality that can retard or negate the cationic cure mechanism.
It would be desirable for epoxy compositions to be toughened on the one hand and remain curable—rapidly and at low temperature—on the other.